User role validation (initially forked from keycloak\examples\providers\authenticator)

Provide the possibility to authorize only users who have specified role.

Example authentication flow:

1. Build a package with : mvn clean package

2. copy artifact from target folder to deployments folder of KeyCloak Server, in case of standalone : KEYCLOAK_DIR/standalone/deployments

3. add new "execution" in authentication flow under "Username Password Form" (or other form which authenticates user)

4. configure the "execution" role - Actions -> Config